Rapid advances in processing of information have placed ever increasing demands on the display systems which transfer critical information to the decision makers in a variety of settings. One serious limitation has been in the area of the quality of display systems meeting the demands of high frame rates and high resolution with compact, cost effective, and efficient optical sources.
Two basic principles, light-modulation and direct emission, are used in video projectors. In a light-modulation design, a beam of light passes through an optical array which is capable of switching individual display elements (pixels) on or off. Liquid crystal display (LCD) panels are common light modulators. Other, more exotic modulators, such as oil films and deformable micro-mirrors, are also available. Direct-emission projectors emit their own light. The most common direct-emission device is the CRT projector, used in home TV projectors and in high power versions for large screen industrial use. Direct-emission projectors are inherently simpler than light-modulation projectors. Conceptually, the direct emission projector consists only of a controllable light source and optics. The evolution of the CRT projector illustrates how, owing to its inherent simplicity, a direct-emission display mechanism can be readily produced and later scaled-up to higher brightness and resolution levels.
As the resolution requirement of high definition television (HDTV) and computer displays increase, CRT-based projectors reach some basic physical limits. A breakthrough is necessary to extend the resolution and brightness limits of this mainstream projection technology. Unfortunately, there are very few viable direct-emission technologies, so designers have looked for other ways to project video and computer information. The result is a host of light-modulation projectors using many different techniques. However, projectors of this type share a set of common problems. One of them is the need for an efficient light source which maximizes the amount of light that is passed through the modulator. Also, the modulation device must be scalable to a reasonably small size. The design issues associated with these needs are difficult and complex, and in many cases they directly limit the achievable results.
To better understand the nature of limitations associated with previous technology, it becomes instructive to review briefly the operating characteristics of four principal contenders in the video projection arena. In order of review, these can be identified as CRT projectors, light valve projectors, passive-matrix panels and active-matrix panels.
CRT Projectors: Projection CRTs are similar to conventional monochrome CRTs, except that they are operated at much higher beam currents. Color systems are built using three independent CRT systems, each with its own lens. The user must make periodic convergence adjustments to bring the three beams into color registration. Attempts to produce single-lens projectors, where the three color tubes are internally converged at the factory, have not been widely accepted.
Light Valve Projectors: Light valve projectors have been developed to overcome some of the deficiencies of the CRT projectors. An advantage of light valve systems is that the light source and modulating element are decoupled. Light valve projectors based on the electron beam oil film light valve were developed over 25 years ago. In such systems, an oil film is used as the image source. Intensity of the projected light is controlled by the amplitude of the deformation of the oil film. However, they are very complex, bulky, expensive, and are difficult to set up and maintain.
An alternative to the oil film approach is provided by the LCD light valve. Here the LCD matrix is not used as a simple shutter that reduces the input light, but a stimulus responsive to the input signal is used to activate the LCD material, which then is coupled to a separate output light source. Extremely high resolutions (5000.times.7000) have been achieved via the LCD approach. However, due to thermal inertia, the writing rate is very slow at this resolution, requiring 3.5 minutes to update a display using two 40 milliwatt lasers. Ultra high resolution is thus offset by very slow writing speed and very high cost.
Passive-Matrix Panels: A simpler method for using an LCD panel as a light modulator is that of interposing the panel between a light source and the projection optics. Such panels can be activated by either of two approaches, either passive-matrix and active-matrix. In both passive and active drive technologies, the LCD cells are arranged in a matrix of rows and columns, and are driven by row and column driver circuits. In a passive-matrix drive system, the LCD cell alone exists at each intersection. A time-multiplexing scheme is used to energize each of the LCD cells in the matrix. Unfortunately, the slow response time of passive-matrix panels makes them unsuitable for displaying quickly changing information such as television signals. Also, crosstalk between LCD cells is a significant disadvantage.
Active-Matrix Panels: An active-matrix panel contains a switching device such as a thin film transistor (TFT), and a storage element (capacitor), in addition to the LCD cell at every LCD site. Each switch/capacitor acts as a "sample-hold" (S/H) circuit for the briefly appearing pulses from the multiplexed drive system. Each LCD cell, driven by its own S/H circuit, is thus decoupled from the other LCD cells, minimizing crosstalk. Furthermore, active-matrix LCD cells can be formulated to respond quickly. Update rates under 50 milliseconds are easily achieved with active-matrix panels. However, active-matrix panels are not easy to manufacture, requiring an impressive range of challenging technologies. The overall yield is the product of a series of process steps, at each of which losses typically occur.
The use of lasers in the production of images, and in particular, the production of a sequential set of electrical signals which represents an original picture for direct display through the use of lasers ("video imaging") is known in the art. See for example, U.S. Pat. Nos. 3,549,800 and 3,721,756 (Baker); 3,571,493 and 3,621,133 (Baker et al.); 3,737,573 (Kessler); 3,818,129 (Yamamoto); 3,958,863, 3,977,770, and 3,994,569 (Isaacs et al.); 3,636,251 (Daly et al.); and 4,720,747 and 4,851,918 (Crowley). See also, Taneda et al., "High-Quality Laser Color Television Display," Journal of the Society of Motion Pictures and Television Engineers, Volume 82, No. 6 (1973); Taneda et al., "A 1125 Scanning-Line Laser Color TV Display," SID International Symposium and Expedition (1973); and Yamamoto, "Laser Displays," Advances and Image Pick-up and Display, Academic Press, Inc., Vol. 2 (1975); and Glenn, "Displays, High Definition Television and Lasers," SID International Symposium and Expedition (1993). For general references to video imaging, see U.S. Pat. Nos. 3,507,984 (Stavis); and 3,727,001 (Gottlieb).
None of the above mentioned patents relate either to diode pumped microlaser technology or to laser array based approaches. Relevant to the present invention, research is ongoing into a variety of red, green and blue (RGB) solid state laser projection technologies. Such work encompasses electrically pumped blue/green semiconductor lasers, nonlinear mixing or frequency doubling of infrared lasers, and infrared-diode-pumped solid state upconversion lasers. All of this work, however, is concentrated on singular red, green and blue sources which, individually, are capable of providing sufficient power at one or other of the colors to provide a brightly illuminated display as opposed to the array approach of this invention as described below.
Possible applications for improved laser projection display systems are for those who need full multimedia integrated projectors. Examples are corporate, industrial, and institutional organizations which make presentations to groups ranging in size from 2 to 500 people. Such users demand good image quality, full motion capability, and integrated sound for their computer and audio/visual (A/V) presentations.